1. Technical Field
The present invention relates to a phosphor substance used for EL (electroluminescence) devices, and more particularly to an EL phosphor laminate thin film and an EL device.
2. Background Art
In recent years, thin-film EL devices have been increasingly studied for compact or large yet lightweight flat display panels. A monochromatic thin-film EL display using a phosphor thin film comprising manganese-doped zinc sulfide for yellowish orange light emission has already been practically used in the form of a double-insulation structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. Referring to FIG. 2, a lower electrode 5 is formed in a predetermined pattern on a glass plate serving as a substrate 1, and a first insulating layer 2 in the form of a dielectric thin film is formed on the lower electrode 5. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer (dielectric thin film) 4 are sequentially formed. An upper electrode 6 is formed on the second insulating layer 4 in such a predetermined pattern as to form a matrix with the lower electrode 5. Usually, the phosphor thin film has been annealed at a temperature lower than the strain point of the glass substrate for luminance improvements.
Recently, a structure using a ceramic substrate for the substrate 1 and a thick-film dielectric layer for the insulating layer 2 has also been put forward. Since this structure uses the ceramic material such as alumina as the substrate, it is possible to anneal the phosphor thin film at high temperature, thereby achieving luminance improvements. Since this structure uses the thick-film dielectric layer for the insulating layer, the structure also features the ability to provide a panel having high resistance to dielectric breakdown and high reliability as compared with EL devices using a thin film for the insulating layer.
To accommodate well to personal computer displays, TV displays and other displays, color displays are absolutely needed. Thin-film EL displays using sulfide phosphor thin films are satisfactory in reliability and environmental resistance, but are now considered unsuitable for color display purposes, because the properties of EL phosphors for emitting the three primary colors of red, green and blue are less than satisfactory. Candidates for the blue emitting phosphor are SrS:Ce where SrS is used as a matrix material and Ce as a luminescence center, SrGa2S4:Ce and ZnS:Tm, candidates for the red emitting phosphor are ZnS:Sm and CaS:Eu, and candidates for the green emitting phosphor are ZnS:Tb, CaS:Ce, etc., and studies thereof are now under way.
These phosphor thin films for emitting the three primary colors, viz., red, green and blue are poor in light emission luminance, efficiency and color purity, and so color EL panels are still on impractical levels. For blue in particular, a relatively high luminance is obtained using SrS:Ce. For the blue to be applied to full-color displays, however, its color purity is shifted to the green side. Thus, much improved blue emitting layers are in great demand.
To provide a solution to these problems, thiogallate or thioaluminate base blue phosphors having good luminance and color purity such as SrGa2S4:Ce, CaGa2S4:Ce and BaAl2S4:Eu are now under development, as set forth in JP-A 07-122364, JP-A 08-134440, Shingaku Giho EID98-113, pp. 19-24, and Jpll. J. Appl. Phys. Vol. 38 (1999), pp. L1291-1292.
The inventors, too, have made studies of thioaluminate base blue phosphors on the premise that blue EL materials of high luminance are essentially required for the development of full-color EL panels. However, the luminance achieved thus far is at most 100 cd/m2 on 1 kHz driving and is still less than practical.
To provide a solution to the aforesaid problems, there is an increasing demand for a phosphor capable of emitting light at a high luminance, and especially a blue phosphor thin-film material. An object of the invention is to provide an EL phosphor laminate thin film capable of emitting light at a high luminance and an EL device.
This and other objects are achievable by any one of the following embodiments (1) to (11).
(1) An EL phosphor laminate thin film comprising a phosphor thin film and a dielectric thin film which are stacked one on the other, the phosphor thin film comprising a matrix material having the compositional formula:
AxByOzSw:M
wherein M is a metal element, A is at least one element selected from the group consisting of Mg, Ca, Sr, Ba and rare earth elements, B is at least one element selected from the group consisting of Al, Ga, In and rare earth elements, x is 1 to 5, y is 1 to 15, z is 3 to 30, and w is 3 to 30, and
the dielectric thin film comprising an alkaline earth-containing oxide and/or a titanium-containing oxide.
(2) The EL phosphor laminate thin film of above (1), wherein the matrix material contains as a main component at least one compound selected from the group consisting of an alkaline earth thioaluminate, an alkaline earth thiogallate and an alkaline earth thioindate, and M is a rare earth element.
(3) The EL phosphor laminate thin film of above (1) or (2), wherein M is Eu element.
(4) The EL phosphor laminate thin film of above (2) or (3), wherein z and w are such numbers that z/(z+w) is 0.01 to 0.85.
(5) The EL phosphor laminate thin film of any one of above (1) to (4), wherein the matrix material is barium thioaluminate.
(6) The EL phosphor laminate thin film of any one of above (1) to (5), wherein the alkaline earth-containing oxide is a perovskite oxide.
(7) The EL phosphor laminate thin film of any one of above (1) to (6), wherein the alkaline earth-containing oxide is barium titanate.
(8) The EL phosphor laminate thin film of any one of above (1) to (7), wherein the dielectric thin film has a relative permittivity of at least 100.
(9) The EL phosphor laminate thin film of any one of above (1) to (8), wherein the dielectric thin film has a thickness of at least 100 nm.
(10) The EL phosphor laminate thin film of any one of above (1) to (9), wherein the dielectric thin film has been formed by a sputtering technique.
(11) An EL device comprising the EL phosphor laminate thin film of any one of above (1) to (10).
The present invention has been accomplished in the course of phosphor thin-film formation experiments made for the target to bring the luminance of a thioaluminate base blue phosphor substance up to a practical level. The EL phosphor laminate thin film thus obtained achieves an outstanding improvement in light emission luminance over conventional EL phosphor thin films.